For a crane, each of its components for bearing load must have certain safety tolerance to ensure safety of apparatus, operators and properties. Therefore, once a weight to be lifted exceeds the lifting capacity of a crane, it is not allowed to lift such a weight by using such a crane. One option is to simply choose to use a crane of a larger size. However, economically speaking, the cost will be increased considerably if a larger crane is purchased for accomplishing the task of lifting, and furthermore the requirement for the floor supporting the crane will become higher if the tonnage of the larger crane exceeds a certain value, which means very high cost for the treatment of the floor will be involved. In order to solve this problem in an economical way, a method of combine lifting by using a plurality of cranes had been proposed already.
The first way of combined lifting is to carry out the lifting when the plurality of cranes are not connected to each other: 1) the lifting hooks of the cranes are attached directly to the same weight (as shown in FIG. 1b) but the load born by each crane is undefined. 2) the plurality of cranes lift the same weight together by a balance beam (as shown in FIG. 1a), such as the lifting equipment described in patent document US7789253B2. For this technical solution, the load on each crane is defined. In both cases mentioned above, anti-tipping ability and structural stability of each crane is not improved compared. Thus during the lifting operation their balance state is vulnerable to be destroyed.
The second way of combined lifting is to use one crane having two booms to lift an object, for instance, the lifting equipment illustrated in patent document CN1899949A. For example, the two parallel booms are connected by a horizontal connecting component, e.g. TEREX-DEMAG CC8800-1 Twin, or the two booms are connected to form an A-shaped united boom, e.g. Sarens SGC-120 ring-rail crane.
The third way of combined lift is to connect two cranes at their respective top of the booms by a short balance beam, e.g. the lifting equipment described in patent document GB19670039354.
The above three ways of combined lifting are considered to have following limitations: 1) compared with the basic crane, the combination crane does not allow to reduce axial load of the boom of a single basic crane and also to enhance anti-tipping ability and structural stability of the basic cranes; 2) the structure is relatively complex; 3) the existing basic crane itself has to be modified greatly to meet the requirement of combined lifting, thereby resulting in relatively high investment.
Therefore the present invention is to seek for a lifting equipment capable of overcoming one or more of the above problems of the prior art.